Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
DHW Isolated, drained and discharged

Observation:

Remove and inspect the DHW filter cartridge located toward the left side of the brass manifold behind the pump. Carefully dismantle the cartridge and inspect the flow regulator and filter gauze.
This cartridge acts as a venturi and creates the pressure differential between the two sensing pipes which in turn operates the diverter valve diaphragm.
Note:
Care should be taken to ensure the cartridge is correctly orientated on refitting so that the filter opening is directly above the cold water inlet.
Incorrect positioning can adversely affect the DHW flow rate.